Effect of the coordination environment on the ability of iron to bind/activate N2: a theoretical study with relevance to the nitrogenase mechanism

[Display omitted] Activation of molecular dinitrogen to ammonia (and an accompanying metal-hydride formation) occurs in nitrogenases at an iron-sulfur active site. Iron-sulfur centers normally feature Fe(II) and/or Fe(III) oxidation states, which in general are not known to bind N2 or H+. Two peculiarities of the nitrogenase active site may be responsible for their unusual reactivity: an unusually reduced Mo(III) center in some of the nitrogenases (possibly suggesting an ability to also generate super-reduced Fe), and the presence of a central carbide anion, inverse-coordinated by 6 Fe centers within the iron-sulfur cluster. To try and deconvolute the factors that may allow the nitrogenase Fe to bind molecular nitrogen, we report here a density functional theory (DFT) study on the ability of a series of mononuclear Fe complexes to bind and activate N2. Structures of the type Fe(H2O)nR(N2) were examined, with the formal oxidation state of the iron varied from 0 to +3, with octahedral or tetrahedral geometry, where R = H2O, HO-, CH3-, CH22-, H2S, H-. Iron-dinitrogen complexes only appear straightforward with lower oxidation states (0, +1), and the carbanion ligands appear distinctly better than oxygen or sulfur at facilitating N2 binding as well as activation/weakening of the N-N bond. For Fe(II), higher spin states and carbanion ligation may in fact allow N2 binding as well – thus raising the question whether in nitrogenases (where sulfur and carbanion-ligated high-spin Fe(II) is present) do require Fe(I) in the catalytic cycle – at least from the point of view of initial N2 ligation.